Thermally sensitive resistors, thermistors, are used in various electric circuits for measurement or detection of temperature changes, or for control of circuit operations in the presence of temperature changes. These thermistors are manufactured as small discs, usually 0.02 inch to 0.8 inch in diameter and 0.01 inch to 0.2 inch thick, from pressed and sintered metal oxide blends. The flat sides of these thermistor discs or wafers are generally coated with a thin electrode material, to which electrical conductors are attached by various means, followed by sealing the whole in an appropriate, protective coating material.
Nagata, in U.S. Pat. No. 3,820,239, sealed platinum lead connected, 0.04 inch diameter by 0.02 inch thick thermistors, with fused glass at about 800.degree. C. Such a coating process provides an inflexible coating, requires platinum type leads, and could result in thermal shock, leaching and oxidation effects on the thermistor. Dankert, in U.S. Pat. No. 3,839,783, taught sealing copper lead connected 0.1 inch diameter by 0.03 inch thick thermistors with an aluminum oxide filled epoxy resin having shrinkage properties, which when cured, presumably at from about 100.degree. C. to 250.degree. C. over several hours, tightly held the contacts in place without a high temperature soldering step. This highly filled coating would still be rather inflexible, and require substantial heating of the assembled thermistor. In Dankert, thermistor wafers, held by spring tension of the leads, were dipped into an electrically insulating, thermally conducting liquid bath and then permitted to cure. Such a process could also allow substantial drainage of the liquid resin from the thermistor wafer edges before gellation and final cure. While several dip and cure cycles could be used, there is a need for a simple, low temperature, single cycle, inexpensive thermistor coating method, which would provide flexible encapsulation not subject to cracking or edge withdrawal.
Lucey, in U.S. Pat. No. 4,282,269, attempted to provide thick, low temperature curable coatings for tubular ceramics and various types of capacitors by using a specific, ultraviolet (U.V.) radiation curable resin system, selected from an acrylic urethane resin or a diacrylate ester of a bisphenol A epoxy resin, both containing either zinc borate, calcium metaborate or barium metaborate, and a photoinitiator, such as acetophenone. The epoxy formulation included a major amount of a nonvolatile diacrylate ester of a bisphenol A epoxy resin, minor amounts of 1,6-hexanediol diacrylate diluent, and photoinitiator, and up to 31 wt.% of zinc borate. The use of zinc borate allowed ultraviolet radiation cure times of from 10 seconds to 50 seconds and provided coatings generally from about 0.005 inch to 0.07 inch thick. The coating could be applied by spraying, painting, dipping or roll-coating, and then cured by passing the coated component under an ultraviolet type lamp on a conveyor system, with about an 11 second to 12 second radiation exposure time. Once initiated, the cure could continue in the dark. Such coatings were still not as flexible and crack resistant as desired nor did the method taught provide outstanding edge coverage of the coated component.
Photosensitive compostions have been widely used in other areas. Sattler et al., in U.S. Pat. No. 4,154,896, used polyester resin copolymerized with acrylic monomer, and containing U.V. photoinitiators, as insulating coating compositions for steel laminations of transformer and generator cores. Sattler et al. taught polyester compositions containing: (1) polyols selected from diols such as 1,2-propylene glycol or triols such as tris(2 hydroxyethyl)isocyanurate, (2) organic dibasic acids selected from both aliphatic dicarboxylic acids, such as maleic or fumaric acid or anhydride, and aromatic dicarboxylic acids, such as terephthalic or isophthalic acid or anhydride or dimethyl terephthalate acid ester, (3) ultraviolet radiation photoinitiator and (4) catalyst. This polyester was then copolymerized with from about 10 wt.% to about 60 wt.% of an acrylate monomer, preferably having at least two acrylic groups, such as hexanediol diacrylate; neopentyl glycol diacrylate; or trimethylol propane triacrylate, to provide fast curing, thermally stable, strong coatings. Simple monoacrylates, such as methyl acrylate or ethyl acrylate were not found useful. More complicated monoacrylates, such as 2-ethyl hexyl acrylate; 2-methoxy ethyl acrylate; or 2-phenoxy ethyl acrylate could be used up to 10 wt.% of the acrylate component. These compositions had good metal wetting properties and abrasion resistance, and provided pin hole free films about 0.0005 inch to 0.005 inch thick. The compositions were continuously coated with a roller, spray, or dip means, onto steel sheet and passed under ultraviolet radiation cure means at speeds of up to 400 ft./min.